The α7 nAChRs are rapidly desensitizing ligand-gated ion channels that are abundantly expressed in the cerebral cortex and the hippocampus, a limbic structure intimately linked to attention processing and memory formation. α7 nAChRs modulate neurotransmitter release and are responsible for direct fast excitatory neurotransmission. At the cellular level, activation of α7 nAChRs can regulate interneuron excitability, modulate the release of excitatory and inhibitory neurotransmitters, and contribute to neuroprotective effects.
Several lines of evidence indicate that impaired attention and cognition, which are characteristic of neurological and psychiatric disorders such as Alzheimer's disease (AD), schizophrenia, Parkinson's disease (PD), multiple sclerosis, attention deficit hyperactivity disorder (ADHD), mild cognitive impairment (MCI), age associated memory impairment (AAMI), may involve degeneration or hypo-function of cholinergic input. Moreover, genetic linkage has identified α7 AChRs as a predisposing factor related to sensory gating deficits. Thus, targeting the α7 nAChRs represents a therapeutic strategy for ameliorating cognitive deficits associated with neurodegenerative and neuropsychiatric diseases.
A number of reports also suggest that α7 nAChRs mediate protection against neurotoxicity induced by amyloid beta and excitotoxic insults. Peripherally, α7 nAChRs are expressed in macrophages and their stimulation is essential for inhibiting the release of proinflammatory cytokines (e.g. TNF-α, IL-1) via the cholinergic anti-inflammatory pathway which is triggered in response to signals from the vagus nerve. Thus, the clinical use of positive modulators of the α7 nAChRs could also represent a strategy against inflammatory diseases.
A growing body of evidence indicates the role of the alpha7 nicotinic receptor subtype in neuropathic pain. Both agonists and positive allosteric modulators have been shown to play an important role in chronic inflammatory and neuropathic pain signaling and to attenuate neuropathic pain in preclinical models.
Selective positive allosteric modulation (PAM) of the α7 nAChR is a recently proposed therapeutic approach for treating these disease states. A key advantage of this approach is that modulation only occurs in the presence of endogenous agonist thereby preserving the temporal and spatial integrity of neurotransmission. Several different profiles have been described for PAMs of the α7 nAChR ranging from Type I modulators that predominately affect the peak current and may also increase channel affinity for the agonist, to Type II modulators that affect the peak current, delay the desensitization of the receptor and may reactivate desensitized receptors. Several PAMs have been described in the literature with some Type I examples including 5-Hydroxyindole, NS-1738, Ivermectin, Galantamine and Genistein; Type II examples including PNU-120596, TQS and A-867744 and some intermediate examples being SB-206553 and JNJ-1930942. All PAMs demonstrate enhanced receptor responses to the endogenous ligands acetylcholine and choline, as well as to nicotine and other agonists.
The present invention seeks to address some of the shortcomings of the prior art therapeutics and is directed to a new class of compounds which exhibit positive modulation of α7 nAChR.